Advanced Ni-base superalloys comprising a mixture of .gamma. and .gamma.' phases are currently isothermally forged at relatively slow strain rates and temperatures below their .gamma.' solvus temperatures. Forging is typically followed by supersolvus annealing. This forging method tends to minimize forging loads and die stresses, and is designed to avoid fracturing the items being formed. It also permits superplastic deformation of these alloys in order to minimize retained metallurgical strain at the conclusion of the forging operations. Other forging methods for Ni-base superalloys have also been recently proposed. One method employs supersolvus forging at slow strain rates followed by supersolvus annealing to control grain size in the forged article, see Ser. No. 08/271,611 filed on Jul. 7, 1994. Other recently proposed methods are designed to impart predetermined levels of metallurgical strain to forged microstructures for the purpose of grain size control, as are disclosed in co-pending patent applications Serial No. 08/298,862, filed on Aug. 31, 1994, and Ser. No. 08/367,635, filed on Jan. 1, 1995. These forging methods also typically employ supersolvus annealing, or a combination of subsolvus and supersolvus annealing. These annealing steps are typically performed in air at temperatures in the range of 1050.degree.-1200.degree. C. (1925.degree.-2192.degree. F.).
Applicants have observed that subsolvus and supersolvus annealing steps commonly used in these forging methods may produce undesirable microstructural changes in Ni-base superalloys. Applicants have observed that when these annealing steps are done in a decarburizing and oxidizing atmosphere, such as air, the growth of abnormally large grains often occurs in a region near the alloy surface. The terms "abnormally large grains" or "abnormal grain growth" or similar temps, as used herein, are defined as grains, or grain growth that results in grains, that are significantly larger than those of the surrounding microstructure, usually by an order of magnitude or more. As used herein, the term "surface region" refers to a region near the surface of a forged article that may be affected by the depletion of species such as carbon and aluminum during forging and/or annealing by the processes described herein, whose depth depends on the composition and morphology of the particular Ni-base superalloy being considered, the forging and/or annealing atmosphere, the duration of the forging and/or annealing steps and other factors. The surface region may extend as much as a millimeter or more below the surface depending on the factors listed.
In the case of subsolvus annealing in air, the surface region of forged articles is depleted in aluminum through oxidation, which in turn depletes the volume fraction of .gamma.' in the surface region. Subsolvus annealing in air is also decarburizing, producing a reduction in the volume fraction of carbides and carbonitrides in the surface region. Annealing above the .gamma.' solvus temperature also depletes the aluminum in the surface region through oxidation, but does not result in the depletion of the volume fraction of .gamma. at the surface because all the .gamma.' is in solution during the anneal. However, supersolvus annealing is also decarburizing, producing a reduction in the volume fraction of carbides and carbonitrides in the surface region. Reductions in the volume fraction of either carbides/carbonitrides or .gamma.' in the surface region tends to create an imbalance between grain growth and boundary pinning driving forces resulting in the growth of abnormally large grains. These same processes are also thought to occur in the surface region during subsolvus or supersolvus forging processes, depending on the factors described above.
While it is common to remove a portion of the surface region of a forged article both prior to and/or after annealing, such as by shaping a forged article prior to annealing, or by performing final machining after annealing, large grains formed in the surface region may extend into the forged articles to depths such that they are not completely removed during these operations. If this occurs post-forging/pre-annealing, the subsequent annealing provides an environment that is conducive to further grain growth, such that the grains may reach a depth that they will not be removed by ordinary post-annealing material removal operations and will be available to affect the properties of the finished article. If large gains grow to a depth that they are not removed after annealing, the result is the same, and abnormally large grains will be found in the final forged article. Unremoved large grains can reduce the low temperature strength, low cycle fatigue (LCF) resistance and other properties of Ni-base superalloy forgings.
Therefore, it is desirable to provide a forging method that reduces the tendency for abnormal grain growth in the surface region of forged articles.